1. Field of the Invention
The present invention relates generally to a light mixing box and a colorimeter employing the same and, more specifically, it relates to a light mixing box employing white ceramics or white resin and a colorimeter employing the same.
2. Prior Art
Conventionally, a reflector for a flash apparatus has been developed mainly for photography in order to enhance reflectance and directivity by using a mirror surface of a parabolic curve. Meanwhile, diffuse illumination has been required in various fields. The diffuse illumination generally lowers the illumination efficiency and makes the illuminating apparatus tend to be large. In some fields, a compact diffuse illumination is required. One of such fields is that of a colorimeter. A normal colorimetry is effected by using a spectrophotometer. However, the spectrophotometer is a large and expensive apparatus, which can not be readily used in general. Therefore, a handy type colorimeter has been developed and used widely in the fields of color conditioning in manufacture, quality control of printed matters, medical inspection, cosmetology, and the like. The object to be measured must be illuminated by the ideal diffused light in order to enhance the precision of colorimetry. As for a measuring apparatus used in the field, measurement should be carried out in a short time period with bright illumination in order to avoid influence of outer light. Therefore, a flash apparatus is used as a light source. A reflector, conventionally a mirror surface reflector, is used therewith to enhance the efficiency of light. As the discharge tube such as xenon tube is used in the flash apparatus, the discharging path in the tube changes every time. This means that the position of the light source moves every time.
FIG. 1 shows a reflector 1 having a paraboloid revolution, and a flash light source 2. The central axis O of the flash light source 2 is located at the focal point of the paraboloid curve of the reflector 1. The light path of the flash light source changes every time dependent on the distribution of excitation of the gas filled in the flash light source. In FIG. 1, the thick solid line shows the path of the light generated from the point A, while the dotted line shows the path of the light generated from the point B. The flash light source 2 can not be regarded as a point source in relation to the size of the reflector 1. Therefore, when the light generated from the light source 2 is reflected by the reflector 1, the reflected light from the point A differs from the reflected light from the point B, depending on the direction of emission, as shown in FIG. 2. For example, in the case of the light reflected in the direction of .phi., the amount of the reflected light becomes as shown by a, c and b, corresponding to the points of light generation A, O, B, respectively. Therefore, the fluctuation in the flash light source causes variation of light amount in the emitting direction of the reflected light, affecting the measurement.
In order to solve the above problem, a diffuse illumination light source is implemented by providing a transmitting diffusion plate such as white acryl plate, frosted glass plate, and the like in front of the light source in a handy type colorimeter shown in FIG. 3. However, the amount of illumination is decreased due to the transmittance characteristics of the diffusion plate. For example, according to the spectral transmittance characteristics of the white acryl plate, the transmittance of the wavelength less than 400 nm is low, so that when it is employed as a light source of a colorimeter, the resolution of blue will be degraded. If the white acryl plate is placed near the flash light source, the plate may possibly be deformed or the color of the plate may be changed due to the power of the light source.
Conventionally, in the field of integrating sphere, powder coatings of barium sulfate, zinc oxide and magnesium oxide are used as materials having no wavelength selectivity, high reflectance and high diffusiveness. However, these materials have problems in the surface adhesiveness and moisture resistance, so that they are not the optimum material for a reflector in the flash apparatus.
As an another example of the prior art, a conventional integrating sphere employed in the colorimeter or other measuring apparatuses is shown in FIG. 4. A perfect diffuse surface 22 having high reflectance and no wavelength selectivity is formed by applying powder of zinc oxide, magnesium oxide, barium sulfate or the like on the inner wall surface of the upper hemisphere 21a and the lower hemisphere 21b. Since the adhesiveness of the powder is weak, it may peel off when an impact is applied thereon. Therefore, the integrating sphere should be carefully handled. A binder may be mixed with the powder to be applied in order to enhance the adhesiveness. However, the performance of the integrating sphere is degraded due to the spectral reflectance characteristics (especially wavelength selective absorption) of the material serving as the binder, and the binder may be turned yellow by the ultraviolet rays. In manufacturing, the reflected layer must be thick in order to enhance the reflectance. Therefore, a number of layers of the powder must be applied to the metal surface. This makes the manufacturing difficult and it is not suitable for massproduction.
FIG. 5 shows a conventional integrating sphere used in a handy type colorimeter which is made in consideration of the impact resistance. Since it is a handy type, the integrating sphere must have high impact resistance and a simple and compact structure. Therefore, the inner surface 29 of the sphere 21 is coated with a delustered type white coating film which is strong. According to the characteristic of the spectral reflectance of the white coating of the inner surface 29, it appears that the reflectance in the wavelength shorter than 420 nm is degraded. Therefore, it does not satisfy the required quality of the integrating sphere, i.e., "it should have reflectance characteristics having no wavelength selectivity". In addition, since the diffusion reflectance characteristics of the delustered white coating of the inner surface 29 differs entirely from the perfect diffusion, the characteristics must be compensated by a diffusion transmitting plate 30 for improving diffusion characteristics. The diffusion transmitting plate reduces the intensity of the illumination.
In a simple integrating sphere employed in a conventional handy type colorimeter and the like, delustered white coatings are employed because of the strength of the coat. However, in that case, the reflectance characteristics of the light in the range of wavelength shorter than 400 nm is inferior, so that required amount of light for illuminating the sample can not be obtained. In a normal integrating sphere, white powder of zinc oxide, magnesium oxide, barium sulfate or the like is applied. Although the reflectance characteristics of the same is superior, it is weak against impact.
As a further example of the prior art, a conventional integrating sphere in a colorimeter of diffused illumination vertical receiving system is shown in FIG. 6, which integrating sphere has the function of removing specularly reflected light.
The vertical light receiving system means that "the angle of inclination of the light reflected from the sample surface from the perpendicular of the sample surface is no more than 10.degree. ". The system shown in FIG. 6 is the 8.degree. light receiving system in accordance with the above definition. In the example shown in FIG. 6, the light from the light source 48 is diffused by the integrating sphere 41 so as to uniformly irradiate the sample 43 and the direct light from the light source 48 is prevented from irradiating the sample 43 by a baffle 49B. The light receiving opening A is inclined by 8.degree. from the vertical direction so as not to receive the specularly reflected light from the sample. A light trap 41A which absorbs specularly reflected light is provided in the direction of reflection. The advantage of this system is that the specularly reflected light from the sample is perfectly avoided, since the light receiving opening A is positioned in relation to the opening portion of the light trap 41A so as not to receive the specularly reflected light. A first disadvantage of the system shown in FIG. 6 is that the opening A can receive no reflected luminous flux in the vertical direction (0.degree. direction) of the sample 43, but it can receive the luminous flux in the direction inclined by 8.degree. from the vertical direction, so that the information of the light in the vertical direction (0.degree. direction), which is originally desired, can not be obtained. A second disadvantage is that in order to obtain the reflected light from the specified region of the measured sample 43 through the light receiving opening A, a collimate lens K must be used and a light trap is required. When a collimate lens K is employed, there are various disadvantages. For example, it is difficult to make the apparatus compact, the assembly becomes troublesome, the apparatus becomes expensive, and it is not suitable for a handy type apparatus.
The center of the light receiving opening A may be provided at the top point P of the integrating sphere so as to place the same in the perfect vertical direction to the measured sample 43, whereby only the vertical reflected light from the sample is measured by a collimate lens. In this structure, since the light receiving opening is provided, there is no wall of the integrating sphere which is the source of the specularly reflected light from the sample to the opening. However, since the reflected light from the sample and from the vicinity of the sample is reflected by the collimate lens and directly irradiates the sample again, the specularly reflected light from the sample 43 is mixed with the measuring light, causing errors in the measurement.
A conventional example of an integrating sphere in a handy type diffusion illumination vertical light receiving system colorimeter (chromameter) is shown in FIG. 7. It differs from the integrating sphere of FIG. 6 in the following points. Namely, the region of the sample 43 to be measured is limited by a cylinder 42 whose axis is the central line of the limiting opening B. The collimate lens K of FIG. 6 is eliminated. The integrating sphere 41 is divided into the upper and lower chambers. A diffusion transmitting plate 50 is provided between two chambers to improve the diffusion illuminating characteristics. In this system, the light illuminating the sample has less orientation but a large quantity of light. However, as for the light emitted from the region (K--K') of the diffusion transmitting plate 50 and reflected on the sample surface which can be seen from the light receiving opening A, the light enters the light receiving opening A as a specularly reflected light from the sample. Therefore, the specularly reflected light from the sample can not be eliminated.
A still further example of the prior art is shown in FIG. 8, in which a reference opening C is provided at a position close to the measuring opening A in order to take the reflected light from the inner surface D of the integrating sphere 51 adjacent to the sampling opening S of the integrating sphere 51. A collimate lens K is provided on the monitoring light axis which goes through the center of the reference opening C and the center of the region D of the wall surface of the integrating sphere 51, so that only the light from the region D of the wall surface is monitored. Therefore, only the light parallel to the light axis of the collimate lens K passes through the collimate lens K to reach the monitor light receiving sensor S2, although the reference surface D is illuminated from all the directions in the integrating sphere 51 and reflects light to all the directions.
Consequently, the amount of the monitor light is small and the S/N ratio of the light measurement is low, so that the variation of the light source can not be effectively corrected. Since the reference opening is approximately opposite to the sampling opening, a collimate lens is used so that the reflected light from the sampling opening do not enter the reference opening. The region of the inner surface of the integrating sphere which is viewed from the collimate lens through the reference opening should not be overlapped with the sampling opening. Therefore, the smaller the apparatus becomes, the smaller becomes the region of the inner surface of the integrating sphere viewed from the reference opening, whereby the monitoring light measurement becomes difficult.
An optical fiber may be used instead of the collimate lens. However, since the optical fiber transmits only light emitted in a certain range of angle, and every optical fiber has different range of light receiving angle. Therefore, in order not to receive the light from the surface to be measured, considerable margin is required in design for positioning and for adjusting the direction of the light axis of the optical fiber. Consequently, it is difficult to make the apparatus compact. A cylinder which limits the range of measurement may be provided projecting in the integrating sphere from the reference opening so as to prevent entrance of the light from the surface to be measured. However, in this case, the integrating sphere becomes far apart from the ideal shape and the light amount can be increased.